That's what he's saying. You're first becomes at best, a tefillas nedava, in retrospect. Your second one was done properly and conts as mariv, then the third counts as the makeup for mincha. When he says "marvi'ach," that doesn't include the benefits of a tefillas nedava; he means benefits of fulfilling a halachic requirement.
Not sure I understood your question correctly, but that's what I think.
I was asking something based on an assumption. I wanted to know what the comparison of the gemaras case is to the case of forgetting yaaleh vyavo.
I thought that when you forget yaaleh vyavo the original tefillah was good and you just need to add yaaleh vyavo which happens to need another tefillah in order to be said.
So over there you won't be adding anything if you're mashlim after Rosh chodesh since you're tefillah on Rosh chodesh worked for whatever you said, just without yaaleh vyavo.
Or in other words I thought the tefillah on Rosh chodesh doesn't turn into a nedava. But the tashlumin afterwards does turn into a nedava (except for yaaleh vyavo).
On the other hand by the case of forgetting shabbos mincha, the first maariv turns into a nedava. Therefore the third maariv is now really the tefillah for shabbos. So it's considered you ARE saying something you haven't said before.
If that's so, then it's possible that R' Alfes would hold you have to daven a third time but by Rosh chodesh you would not have to daven over.
If I'm making a mistake please let me know.